1. Field of the Invention
This invention relates to a heater temperature control circuit suitable, for example, for carrying out temperature control of a heater for fixing a toner image on a recording material in laser printers.
2. Description of the Related Art
In laser printers, temperature control of a heater for fixing a toner image on a recording material is conventionally carried out by a heater temperature control circuit as shown in FIG. 5. The shown heater temperature control circuit comprises a comparator 102 for comparing, with a fixed reference voltage V.sub.ref, a divided voltage V.sub.in divided by a thermistor 100 mounted on a heater and a resistance R13. The heater is turned on and off on the basis of the results of comparison. In the above-described arrangement, however, a set value (divided voltage V.sub.in) of a control temperature (a target temperature) for the heater is determined by a resistance value of the resistance R13 used. The resistance R13 needs to be replaced by another having a different resistance value when the set value is changed after the assembling of the resistance. The replacement of resistances takes a great deal of trouble. Furthermore, a plurality of resistances R11, R12 and R13 are required in order that the heater temperature control circuit may have a plurality of set values of the control temperature, as shown in FIG. 5. In the arrangement of FIG. 5, the resistance R11 is used for the control temperature of 160.degree. C., for example. The resistance R12 is used for the control temperature of 180.degree. C. The resistance R13 is provided for detecting an abnormal condition of the heater. An increase in the number of resistances increases a manufacturing cost of the heater temperature control circuit. Additionally, a complicated control manner is required when the set value of control temperature is changed minutely. In this case, a large number of resistances having different resistance values need to be provided. This would be substantially impossible. A resistance R17 in FIG. 5 is provided for giving a function of hysteresis to the circuit.
To solve the above-described problems, the prior art has provided an arrangement that a divided output divided by the thermistor 100 and the resistance R13 is converted to a digital value (digital signal) by an A/D converter, which digital value is read by means of software in a microcomputer. Based on the digital value read (detected temperature), the microcomputer on/off-controls the heater by means of software. In this case, since a determination or reference value for the divided voltage V.sub.in is changed in a software manner, a complicated temperature control manner can be accomplished without use of a large number of resistances. However, an A/D converter is an expensive device and accordingly, the manufacturing cost of the heater temperature control circuit is largely increased.